1. Field of the Invention
The present invention relates to a power output apparatus and a method of controlling the same. More specifically the present invention pertains to a power output apparatus for outputting power to a drive shaft and a method of controlling such a power output apparatus.
2. Description of the Prior Art
Known power output apparatuses for carrying out torque conversion of power output from an engine and outputting the converted power to a drive shaft include a combination of a fluid-based torque converter with a transmission. In the torque converter, an input shaft and an output shaft of the power are not fully locked. This causes a slip between the input shaft and the output shaft and leads to an energy loss corresponding to the slip. The energy loss is expressed as the product of the revolving speed difference between the input shaft and the output shaft and the torque transmitted to the output shaft, and is consumed as heat.
In the conventional power output apparatus, the transmission is used as a reverse gear for reversing the rotation of the output shaft of the engine and transmitting the reversed rotation, in order to rotate the drive shaft in reverse of the rotation of the output shaft of the engine.
In a vehicle having such a power output apparatus mounted thereon as its power source, at the time when significantly large power is required, for example, at the time of starting the vehicle or running the vehicle on an upward slope at a low speed, a large energy loss in the torque converter undesirably lowers the energy efficiency. Even in a stationary driving state, the efficiency of power transmission in the torque converter is not 100%, and the fuel consumption rate in the known power output apparatus is thereby lower than that in a manual transmission.
In order to solve such problems, the applicants have proposed a system that does not include the fluid-based torque converter but has an engine, a planetary gear unit, a generator, a motor, and a battery and outputs the power from the motor to the drive shaft by utilizing the power output from the engine or electric power stored in the battery (JAPANESE PATENT LAYING-OPEN GAZETTE No. 50-30223). In this reference, however, there is no description of the technique of outputting a power in reverse of the rotation of the output shaft of the engine to the drive shaft with the power output from the engine.